Printing is one of the most popular ways of conveying information to members of the general public. Digital printing using dot matrix printers allows rapid printing of text and graphics stored on computing devices such as personal computers. These printing methods allow rapid conversion of ideas and concepts to printed product at an economic price without time consuming and specialised production of intermediate printing plates such as lithographic plates. The development of digital printing methods has made printing an economic reality for the average person even in the home environment.
Conventional methods of dot matrix printing often involve the use of a printing head, e.g. an ink jet printing head, with a plurality of marking elements, e.g. ink jet nozzles. The marking elements transfer a marking material, e.g. ink or resin, from the printing head to a printing medium, e.g. paper or plastic. The printing may be monochrome, e.g. black, or multi-coloured, e.g. full colour printing using a CMY (cyan, magenta, yellow, black=a process black made up of a combination of C, M, Y), a CMYK (cyan, magenta, yellow, black), or a specialised colour scheme, (e.g. CMYK plus one or more additional spot or specialised colours). To print a printing medium such as paper or plastic, the marking elements are used subjected to electric firing pulses and are “fired” in a specific order while the printing medium is moved relative to the printing head. Each time a marking element is fired, marking material, e.g. ink, is transferred to the printing medium by a method depending on the printing technology used. Typically, in one form of printer, the print head is held stationary and extends in a first direction across the complete width of the printing medium. The printing medium is moved relative to the print head in a second direction perpendicular or substantially perpendicular to the first direction to produce a series of so-called raster lines which extend in the first direction. A raster line comprises a series of dots delivered onto the printing medium by the marking elements of the printing head. It is preferred if the relative movement between the printing head and the printing medium is smooth and continuous but the printing medium may be moved intermittently in the second direction. An encoder linked to the means for moving the printing medium may provide pulses which can be used to synchronise the print head operation with the movement of the printing medium. The above is often described as “page-wide” printing using a page-wide print head.
The combination of printing raster lines and moving the printing medium relative to the printing head results in a series of parallel raster lines which are usually closely spaced. Seen from a distance, the human eye perceives a complete image and does not resolve the image into individual dots provided these dots are close enough together. Closely spaced dots of different colours are not distinguishable individually but give the impression of colours determined by the amount or intensity of the three colours cyan, magenta and yellow which have been applied.
In order to improve the veracity of printing, e.g. of a straight line, it is preferred if the distance between dots of the dot matrix is small, that is the printing has a high resolution. Although it cannot be said that high resolution always means good printing, it is true that a minimum resolution is necessary for high quality printing. A small dot spacing in the slow scan direction means a small distance between marker elements on the head, whereas regularly spaced dots at a small distance in the fast scan direction places constraints on the quality of the drives used to move the printing head relative to the printing medium in the fast scan direction.
Usually, a drive mechanism for moving the printing medium relative to the print head is controlled by a microcontroller or microprocessor, a programmable digital device such as a PAL, a PLA, a FPGA or similar although the skilled person will appreciate that anything controlled by software can also be controlled by dedicated hardware and that software is only one implementation strategy.
One general problem of page wide printing is the formation of artifacts caused by the digital nature of the image representation and the use of equally spaced dots. One source of artifacts can be errors in the placing of printed dots caused by a variety of manufacturing defects such as the location of the marker elements in the print head or systematic errors in the movement of the printing head relative to the printing medium. In particular, if one marking element is misplaced or its firing direction deviates from the intended direction, the resulting printing will show a defect which can run throughout the printing. Similarly, a systematic error in the way the printing medium is moved relative to the printing medium may result in defects which may be visible. For example, slip between the drive for the printing medium and the printing medium itself will introduce errors.
Such errors as described above may result in “banding” that is the distinct impression that the printing has been applied in a series of bands. The errors involved can be very small—the colour discrimination, resolution and pattern recognition of the human eye are so well developed that it takes remarkably little for errors to become visible.
To alleviate some of these errors it is known to alternate or vary the use of marker elements so as to spread errors throughout the printing so that at least some systematic errors will then be disguised. For example, one method often called “shingling” in which multiple passes are made with less than the complete number of marking elements firing at the same time. However, printing dictionaries refer to “shingling” as a method to compensate for creep in book-making. The inventors are not aware of any industrially accepted term for the printing method wherein no adjacent pixels on a raster line are printed by one and the same nozzle. Therefore, from here on and in what follows, the terms “mutually interstitial printing” or “interstitial mutually interspersed printing” are used. It is meant by these terms that an image to be printed is split up in a set of sub-images, each sub-image comprising printed parts and spaces, and wherein at least a part of the spaces in one printed sub-image form a location for the printed parts of another sub-image, and vice versa.
Another method of printing is known as “interlacing”, e.g. as described in U.S. Pat. No. 4,198,642. The purpose of this type of printing is to increase the resolution of the printing device. That is, although the spacing between nozzles on the printing head along the slow scan direction is a certain distance X, the distance between printed dots in the slow scan direction is less than this distance. The relative movement between the printing medium and the printing head is indexed by a distance given by the distance X divided by an integer.
The methods described above often include multi-pass printing. That is that the print head passes over the printing medium in several “passes” in order to print a complete part of the image. Each printing pass only provides the printing of an incomplete image which consists of printed portions and unprinted portions distributed over the printing medium. The multi-passes fill in the parts of the printed image which are missing. The reason for the multi-passes can be that each colour separation of an image is printed in one pass, or that each individual monochromatic image which makes up a complete coloured image (e.g. three—CMY or four—CMYK) is printed by a series of passes.
Multi-pass printing is very common in ink jet printers using a scanning printhead for the fast scan movement and a paper advance for the slow scan movement. Multi-pass printing can have a dual purpose:    1. When the intrinsic resolution of the printhead is lower than the targeted printed image resolution, the printhead cannot print all pixels during one pass. The image is in that case written by “interlacing”. This means that dot lines are printed along the direction of movement of the printing medium in between the dot lines printed during a previous pass.    2. Normally one nozzle is “responsible” for all pixels in one dot line along the fast scan direction. Due to drop misplacements, typical for each individual nozzle, banding will occur. By introducing “shingling” one nozzle will not print all pixels during one pass. During other passes, other nozzles will print pixels not yet printed in that particular dot line by the previous nozzle. Shingling is not used to write images with a higher resolution than the intrinsic head resolution. Shingling spreads the drop misplacement caused by deviating nozzles and paper transport inaccuracies.
When a digital printing press is using a page wide array of nozzles, generally only a movement in the direction of transport of the printing medium exists. In a basic layout the printing medium is just passing only once in front of the print head. Drop misplacement or non-functional nozzles will create a banded image. The single pass concept leaves no room for shingling.
In the European patent applications EP 00 204699 and EP 01 000701 owned by the present applicant, the page wide printing head can move along its nozzle array direction. In this way it is proposed to accomplish shingling by having two arrays positioned in a certain way to print a first pass when text & line art and images (mixed mode) are to be printed. After this pass the first array is shifted over a half nozzle pitch to the previous position where the nozzles of array 2 were writing during the first pass. In the same way array 2 is positioned where array 1 was writing during the first pass. The above system runs at the basic single pass printing speed. Because shingling requires multiple passes, the throughput of this system goes down in relation to the amount of shingling that is done. In a particular embodiment 2 times shingling was used and as a result the throughput went down with a factor of 2. With the same particular embodiment, the system uses 2 arrays of 360 dpi per colour. When using 2 such arrays in one line it is possible to use 2 times shingling or redundancy. The advantage is that shingling is obtained in one pass and that the throughput is unaffected. However, the use of 2 arrays instead of one doubles the cost.
For pure text & line art a single pass with two arrays shifted over a half nozzle pitch will always be faster than using one head to increase the resolution by interlacing in a second pass. A particular embodiment of this concept uses two 360 dpi arrays shifted over a half nozzle pitch to write 720 dpi.
When multiple passes are used, the overall speed of printing is reduced. There is a continuous requirement for improvements in printing methods and printers. In particular, there is a requirement to increase the efficiency of multi-pass printing while providing high quality.
It is an object of the present invention to provide a printing method and apparatus which can provide high resolution printing at high speed.